The artists are best known for their "live art" performance of pillow fighting on Tracey Emin's work My Bed at the Tate Gallery last October.
Basking in the sunshine on Saturday, they strolled across the London landmark wearing only ink slogans promoting artistic freedom, each holding a paw of a giant Panda teddy bear.
Mr Cai and Mr Xi explained that the soft toy was dubbed Tony Bear because the Chinese cannot pronounce the letter L in Blair.
But onlookers may have thought Bare was more appropriate.
Mr Cai, 44, a graduate of the Royal College of Art, and Mr Xi, 38, who studied art at Goldsmiths College, met at art college in China before moving to Britain in the 1980s.
Revolutionary art
Mr Cai had spent three years in a Chinese jail.
One year of his sentence was hard labour for anti-revolutionary thought-crimes.
He found himself in trouble at the age of 16 after listening to Simon and Garfunkel on the radio.
Then he plotted to flee to Russia but his father, a state publisher, informed on him.
Mr Xi caused controversy through his early performance work, which involved running naked in minus 20 degrees through Beijing University campus.
Both married English women, and obtained British passports, and have been living in London since.
'Celebration of modern art'
The pair were arrested for the stunt at the Tate, which they called Two Naked Men Jump into Tracey's Bed, but released without charge.
They claimed to have followed up their bed bouncing at the Tate Modern in May by filming themselves urinating over Marcel Duchamp's Fountain - a ready-made urinal first exhibited in 1917.
They usually claim their antics are in the name of "celebrating the spirit of modern art" as they investigate the political and artistic struggles in society.
They said the demonstration on Saturday was to depict the capital as a battleground for anybody who wanted to play out their dreams and fantasies.